At the present time, conductive pastes such as silver pastes are used for forming electrodes or circuits of electronic components and the like. In addition, the conductive pastes are also used as a conductive adhesive and used for adhesion between the components and the like. As for the characteristics required as the conductive adhesive, there are mentioned, in addition to conductivity, thermal conductivity for releasing heat generated in an electronic component to the outside. In general, specific resistivity and coefficient of thermal conductivity of metal have correlation to each other, and according to the Wiedemann-Franz law, it is expressed by λ=L×T/ρv (in the formula, λ is a coefficient of thermal conductivity of metal; L is the Lorenz number; T is an absolute temperature; and ρv is a specific resistivity). That is, this law indicates that the lower the specific resistivity of conductive film, the higher the coefficient of thermal conductivity is. For that reason, metals having a low specific resistivity, such as silver, are also excellent from the standpoint of thermal conductivity.
Patent Document 1 discloses a conductive paste containing silver nanoparticles having a particle diameter of 100 nm or less, a protective colloid constituted of an organic compound having a carboxyl group and a polymer dispersant, and a solvent. When this conductive paste is baked at 100° C. or higher to remove the solvent, the silver nanoparticles are sintered, whereby a conductive film composed of a metallic bond is formed, and therefore, it is possible to form a conductive metal film close to a bulk. In addition, this patent document describes that in the case where an adherend surface is a noble metal, the silver nanoparticles are sintered on the adherend surface to undergo metallic bonding, and therefore, joining composed of a metallic bond is achieved, and it becomes possible to achieve joining with tremendously low resistivity and high heat radiation properties. Furthermore, in this patent document, after the above-described conductive paste is coated on a one-sided adherend (a so-called substrate or lead frame), the other-sided adherend (a so-called chip) is installed on the coated conductive paste, and the coated conductive paste is sandwiched by the both adherends, followed by heating to achieve adhesion.
However, according to a method of using this conductive paste (adhesive), productivity is low and a use application thereof is also limited. That is, in the above-described method, when the solvent of the coated adhesive is volatilized and dried prior to installation of a chip on the coated conductive paste, even if the chip is installed, the conductive paste does not adhere to the chip. Drying of the adhesive becomes faster when a coating area is smaller and furthermore, when a coating thickness is thinner. In a chip packaging step, there is frequently found the case where after coating the adhesive and then elapsing several hours, chip mounting is performed. In order to respond to such a step, a method of delaying a volatilization rate of the solvent to be used for the adhesive is necessary. In particular, in joining an LED (light-emitting diode) chip, since a joining area is several hundred μm□ (several hundred μm×several hundred μm) or less, drying of the adhesive is particularly fast. In coating of an adhesive by means of dispensing or pin transfer, since the coated adhesive is coated or transferred in a thickness of 100 μm or more, though drying of the adhesive is slow, a coating thickness of the adhesive varies and extrusion of the adhesive or the like is caused. Therefore, coating of the adhesive by means of dispensing or pin transfer is unsuitable in LED packaging in which high positioning precision is required. In addition, because a superfluous excess of the adhesive is coated, the costs become high.
As a high-precision and inexpensive coating method, there is mentioned a screen printing. According to the screen printing, an adhesive can be coated in a thickness of about several ten μm, and as compared with dispensing or pin transfer, it is possible to coat a printing pattern with high precision. In the screen printing, viscosity and rheology of a paste are important. In order to control the viscosity and rheology of a paste, in general conductive adhesives, resins having both adhesiveness and viscosity are used. However, in the case of using a general conductive adhesive, an interface between metal nanoparticles and an adherend surface or a gap between metal nanoparticles, is continuity due to physical contact, and hence, the electrical resistance value and heat radiation properties are lowered. Meanwhile, the metal nanoparticles has a surface protective agent composed of a surfactant chemically adsorbed on the surface thereof, and therefore, by choosing the surface protective agent, printing properties can be ensured. It is to be noted that in a metal nanoparticle paste having excellent printing properties, in general, a large quantity of the surface protective agent is adsorbed on the metal nanoparticles. In this case, since sintering between the metal nanoparticles is hindered, joining is difficult. So far as a paste in which the quantity of the surface protective agent is small relative to the metal nanoparticles is concerned, though it is possible to achieve rheology suitable for screen printing by increasing the metal concentration, in this case, drying after coating the adhesive becomes extremely fast.
It is to be noted that Patent Document 2 discloses, as a resin composition for forming an insulating protective film, a resin composition containing a resin having at least one bond derived from an acid anhydride group and/or a carboxyl group, inorganic fine particles, and a urea-modified polyamide compound and/or a urea urethane. This patent document describes that the printing precision of screen printing or the like is improved. This patent document describes that the urea-modified polyamide compound and/or urea urethane is used as a viscosity modifier. In the working examples thereof, a resin composition containing a resin having a carbonate skeleton, inorganic particles such as silica particles or barium sulfate particles, and the above-describe viscosity modifier was subjected to screen printing and then heated for curing, thereby forming a resin coating film.
However, this patent document does not describe an interaction between the inorganic fine particles and the viscosity modifier.